Moving image encoding normally involves intraframes for intra-frame encoding, and interframes for inter-frame prediction. Intraframes are normally set every 0.5 to several seconds, and the remaining frames are encoded as interframes. When an interframe is encoded, an intraframe or another interframe is used as a reference destination in inter-frame prediction.
Cyclic noise is sometimes caused by such moving image encoding, and the noise is normally called heartbeat noise. Heartbeat noise is noise that is recognized in synchronization with the intraframe appearance cycle, and, because of the heartbeat noise, an image appears to be changing in a constant cycle. Heartbeat noise tends to be generated more often at locations where there are few moving objects, there is little panning of a camera or the like, and inter-frame prediction is valid in interframes.
For example, after an intraframe appears in the first cycle, encoding that does not involve transmission of DCT coefficients, or skipping, is employed with inter-frame prediction. As for the intraframe that appears in the next cycle, encoding that does not involve transmission of DCT coefficients, or skipping, is also employed with inter-frame prediction.
In a case where images subjected to such encoding are viewed, a visual gap is sometimes felt between an inter-frame predicted image group 1 after the intraframe that appears in the first cycle and an inter-frame predicted image group 2 after the intraframe that appears in the next cycle. The subjective image degradation due to this gap is the heartbeat noise described herein.
As a technique for reducing this heartbeat noise, there is a technique for reducing the above described visual gap between the predicted image group 1 and the predicted image group 2 with a smoothing filter. For example, the technique disclosed in Patent Document 1 is a method of removing grain noise that overlaps images captured by a camera or the like, and this method can also be used on heartbeat noise.